In a communication network which is operated with a plurality of terminal units connected to a host computer and to each other, data sometimes collide with each other in the communication line, thereby causing illegible errors.
In order to detect data collisions in a communication system, there is a method of detecting the edges of received pulses for the detection of data collisions; a method of detecting the height of the threshold amplitude of transmitted signals for detection of data collisions; a method of checking the transmission and receiving CRC (cyclic redundancy check) codes for detecting data collisions; and a method of detecting the violation of the code rule of the Manchester code for detecting data collisions.
Among these methods for detecting data collisions, the most widely used one at present is the technique in which the violation of the code rule of the Manchester code is detected, which is used for detecting data collisions, and this technique is disclosed in U.S. Pat. No. 5,012,467.
In this method, a checking is made as to whether a transition occurs within a pre-set time window, and whether a next transition occurs in the same time window. If no transition occurs, there is output a signal which notifies an occurrence of a collision between signals.
That is, transitions at the center of a bit cell of the Manchester code is detected so as to output a trigger signal. This trigger signal is supplied to a pulse generator which has a certain pulse width and which is capable of re-triggering, thereby setting a time window. If the trigger signal does not appear within the time window, then a data collision detection signal is output.
However, in such conventional methods and circuits in which a violation of the code rule of the Manchester code is detected for detecting data collisions, an edge trigger detecting circuit is not used. Therefore, a relatively long delay (88-1100 .mu. seconds) is required for setting the time window. For this reason, an RC analog circuit is additionally required, with the result that the integration density and reliability worsen, and the manufacturing cost is increased.